This invention relates to web handling apparatus. It relates more particularly to a system for helping to maintain control of web tension in such apparatus.
The apparatus with which we are concerned here is used to provide web uninterruptedly to machines which consume the web at high speed, a printing press, for example. We are also concerned with machines such as rewind stands which rewind web uninterruptedly from a source. In both cases, the apparatus is designed to automatically splice the leading end of one web to the trailing end of another.
Thus, in the case of an unwind stand, the apparatus is designed to automatically splice the leading end of a roll of ready web to the trailing end of a depleting roll of running web and to deliver the web continuously in a controlled fashion to the web-consuming machine. In order to accomplish this, the apparatus includes an accumulator or festoon situated between the splicer and the web-consuming machine. When the running web is stopped to splice its trailing end to the leading end of the ready web, there is enough material in the accumulator to supply the needs of the web-consuming machine until the ready web roll is accelerated to running speed after the splice is made.
In the case of a rewind stand, when a roll becomes full, the incoming web is severed from that roll and spliced to a new roll core which is then accelerated up to speed. While the splice is being made, web continues to flow uninterruptedly from the source and is stored in an accumulator. Thus, many of the same problems are encountered in both types of rollstand. For purposes of illustration, we will describe the present invention as applied to an unwind stand which supplies web uninterruptedly to a web-consuming machine, a typical such system being described in detail in U.S. Pat. No. 3,305,189.
Basically, the apparatus includes supports for a pair of web rolls, one of which is running and one of which is at the ready. The running web is conducted into an accumulator and the web material leaving the accumulator travels to the web-consuming machine. The accumulator is comprised of a set of fixed rolls and a movable dancer carrying a second set of rolls. The web is looped between the fixed rolls and the dancer rolls, forming a series of bights. The amount of material in the accumulator is controlled by moving the dancer toward or away from the set of fixed rolls. In other words, as the dancer moves further away from the fixed rolls (i.e. upward), the amount of material in the accumulator increases, and vice-versa. In operation, the dancer has a set bias away from the fixed rolls and is caused to move toward or away from the fixed rolls by changes in the tension in the web.
Usually, web is pulled from the running roll by a pulling roll in the web-consuming machine at a constant rate whose value depends upon the requirements of the web-consuming machine. The running supply roll is braked in a controlled fashion to maintain the proper web tension. In the apparatus illustrated in the above patent, the amount of braking force applied to the running web roll, i.e. the amount of tension imparted to the web, is determined by the position of the dancer in the accumulator which, as noted above, reflects web tension.
As the dancer moves towards the set of fixed rolls (i.e. downward) indicating an increase in web tension, the web braking force is reduced so that the web runs into the accumulator at a faster rate, decreasing web tension and thereby tending to move the dancer up away from the set of fixed rolls. Conversely, if the dancer moves upward, indicating a web tension decrease, the braking force on the running web roll is increased to slow down the rate at which the web enters the accumulator. This increases web tension and tends to move the dancer downward toward the set of fixed rolls.
Thus, during normal operation of the apparatus, the dancer moves about a reference position in an attempt to maintain substantially constant tension in the web. This reference position is selected so that there is enough web in the accumulator to satisfy the needs of the web-consuming machine when the running roll is stopped to make a splice. The arrangement for controlling tension during rewind is similar except dancer position is used to control the drive for the rewind roll instead of a brake.
These prior systems work well at relatively low web speeds. However, at high speeds, on the order of 1500 feet per minute, certain problems develop. For example, the running web supply roll is constantly decreasing in diameter so that its moment of inertia is constantly changing. Accordingly, when a certain braking force is applied, the resulting web tension change depends upon the amount of material remaining on the roll at the time. In practice, the decreasing roll size causes a gradual increase in web tension, with the result that the dancer moves down and reduces the amount of available material in the accumulator.
Some systems do exist which vary the braking force applied to the running web roll in accordance with the amount of material on that roll at any given time. These prior arrangements all measure the roll size and vary the braking pressure inversely with roll size. In one type, the size of the running roll is measured directly by means of a follower arm which engages the circumference of the running roll. In another type, the roll size is measured indirectly electronically by measuring and comparing the angular velocities of the running web roll and a constant diameter "web in" roll to the accumulator which rotates at uniform speed. While those types of prior systems are similar to the present one in many respects, the former is not very accurate at high speeds and the latter is unduly complicated and costly for some applications. Accordingly, it would be highly desirable if a simpler, less expensive, yet accurate web tension control system could be installed on present-day roll stands and rewind stands which are designed to operate at high speeds.